Move files from Books to external hard drive

I need to move files from my iPad to an external HD to free up some storage.  I can connect my HD with no problem and have successfully moved photos.  Books are a problem.  Most of the files in Books are .pdf and books in public domain that I have downloaded from the internet.  Very few books (less than 10) from Apple  bookstore.  I should be able to select multiple books (pdfs) click the share icon then tell the system where to move them. When I get to the “select ” window I can select files but when I select the second file the share icon grays out, even if both files are .pdfs that I created.   I do need to move multiple files, moving them one at a time is not practical.  Any suggestions greatly appreciated.  Thanks in advance.